Drainage Query

[markocosic]

Hi,


I'm looking to drain my garden/swamp in order to

1) use it outside of summer!
2) replace a prefab garage and wasted space with a a proper outbuilding/workshop (red spray paint, approx 50m^2 so subject to BC) under PD.

The garden is the lowest point in the area and runoff from the public highway is a problem.

Untitled

• markocosic
• 15 Oct 2013
• 1

-Can I drain surface water from the public highway into a combined household drain/sewer system?

-Is there an official route is there to adding a highway drain, or having a drain on my property that handles runoff from the highway and then drains into the highway drains?

-Any official way of asking nicely for the highway not to drain into the garden?


Detail:

The large tarmac area to the west of the house (Jermyn Close) has a line drain (orange line) at front of the garages and a street drain (circled in orange) to the southwest corner. Unfortunately the falls (marked in purple) are all away from the drains and into my back garden. The fall is approx 120 mm from the garages to the line marked "jermyn close" and approx 80 mm from that line to the gardens. From North to South generally the fall is approx 100 mm.

Untitled

• markocosic
• 15 Oct 2013
• 1

Untitled

• markocosic
• 15 Oct 2013
• 1

The footpath and the gardens for the flats are 150-300 mm higher than my garden too. It nominally falls away towards the flats, but as there's no drainage there either it tends to either run along the footpath and onto the street or onto their grass before seeping straight back under the footpath through the gravel and through/under the wall. (the bottom of the wall on the garden side is trashed from permanent moisture and frost damage)

There is a drain at the low corner for the footpath which I've recently unclogged, but there's nothing at all for the large tarmac area. It drains via the paving slabs and a muddy trench between the tarmac and my wall, under/through the wall into the back garden. In summer it's ok. In spring, autumn, and winter it squelches.

Untitled

• markocosic
• 15 Oct 2013
• 1

Untitled

• markocosic
• 15 Oct 2013
• 1

Public drains are orange. The static water level in these is quite high: only 80 mm below the surface of the tarmac at that low corner, and 160 mm at the larger drain to the southwest corner.

Household drains are shown in green. A shared soil stack runs across the back garden, along the side path and into a manhole a the front end of this path. This is much deeper than the street drains, approx 1500 mm below the floor level of the house and 1250 mm below that of the tarmac out back. The run is ~30 metres from the very back of the plot to this manhole. Gutter downpipes exit into the same drain so I'm assuming it's a combined system. (who would you ask officially to check?)

It's Cambridge, so flat as a pancake, though I'm lucky enough to be above sea level and on sandy/gravelly clay rather than pure clay! What would you advise to drain the swamp?

Building the outbuilding/workshop is an opportunity of sorts. I'm not averse to using something like these under the foundations or a small patio area between the building and the garden:

http://www.pipelife.com/media/generic/pdfs/eco_systems/Stormbox.pdf

(hardcorelastic boxes:EPS/XPS:concrete raft:lightweight timber framed workshop)

 

[evyxmsj]

Hello!

In reply to your first query: no you can't connect the highway drainage into the combined sewer system, at least not without agreement. They are likely owned by two different authorities and you would need permission to do this.

Generally speaking, it is not a great idea to increase impervious area connecting to a combined system anyway (you get combined sewer overflows).

It looks like the muddy trench could be a filter drain (aka French drain). Have a look if it is full of gravel. If it is, you could try to request maintenance from whomever owns it (you should check if it is the County or not---it could be private). The filter drain should ideally have a pipe from the base or halfway up to the gully.

If it is a filter drain, then you can easily demonstrate an existing drainage status quo'. You could try to replace it with a more robust filter drain (ie with geotextile wrapping). You might be able to get away with putting in a channel/linear drain considering soakaway potential of a filter drain is 0 when groundwater is above the base!

If there isn't a drain at present, then you will need to establish the site owner and convince them that something is needed. This could be a hard sell. It might be easier if you offer to install it, but they will still have to maintain it and also check that additional impervious area isn't going to overload their existing system. You could do some basic calculations of runoff if needs be.

I like the geocells generally, but you might not have much success as it sounds like they would just fill up with permanent water!

 

[markocosic]

Thanks evyxmsj

I should probably clarify boundaries/intentions here. Muddy trench is mine, wall is mine, part fence w/neighbour is mine, and the wall/fence will be coming down in order to erect the outbuilding on/very near to the boundary.


In light of this:

no you can't connect the highway drainage into the combined sewer system, at least not without agreement

Would it technically be drainage of "my land" even if most of the drainage would be runoff onto my land from that tarmac area?

Generally speaking, it is not a great idea to increase impervious area connecting to a combined system

Agreed, hence interest in hooking up to the public surface water drains.

It looks like the muddy trench could be a filter drain (aka French drain). Have a look if it is full of gravel. If it is, you could try to request maintenance from whomever owns it (you should check if it is the County or not---it could be private). The filter drain should ideally have a pipe from the base or halfway up to the gully.

If it is a filter drain, then you can easily demonstrate an existing drainage status quo'. You could try to replace it with a more robust filter drain (ie with geotextile wrapping). You might be able to get away with putting in a channel/linear drain considering soakaway potential of a filter drain is 0 when groundwater is above the base!

No such luck unfortunately. There's the gravel/hardcore under the tarmac and concrete edging for their kerb, then a bunch of mud sat atop the foundations of my wall. The clay pot draining the footpath exists to the southwest, although it's in such bad shape (holed) that a nonzero amount of water from the tarmac currently falls into it from the side but only after it's soaked in.

If there isn't a drain at present, then you will need to establish the site owner and convince them that something is needed. This could be a hard sell. It might be easier if you offer to install it, but they will still have to maintain it and also check that additional impervious area isn't going to overload their existing system. You could do some basic calculations of runoff if needs be.

Cambridge City Council. Up to my boundary is "public highway" (Jermyn Close) then to the north it's "private land" owned by the council over which nobody has a right of access.

Untitled

• markocosic
• 15 Oct 2013
• 1

If you can work out what the legal situation is/make their lives easy/are patient I've found the Council are actually pretty good about sorting "miscellany" out. The land to the rear of the existing garage for example, shown on that map as public highway, was technically "private land" owned by the council over which I had an implied right of access through 20 years continuous use of that garage. It wasn't mine (thanks to an oversight by somebody drawing the right to buy boundary back in the day) but nor was it public highway or of any use to them - so they transferred it to me for zero nominal value with me to pay costs of £400. Very reasonable for a council - but it was me who had to do the legal legwork of finding out what the status was etc and it did take 8 months for them to get around to it!

Do you know what the runoff calculations are?

Does it being a public highway affect matters? Perhaps records that will have been kept about the original highway/drainage design? (maybe it was all meant to drain into the one big drain to the southwest, or they've tarmaced over a drain in that corner/got the levels wrong during one of many patches and refurbishments)

As above, if I can find out what ought to happen I expect Cambridge City Council will be pretty reasonable. (just don't expect them to do the legwork)

(a line drain into that pavement drain would be easy to do if it'll take the flow)

 

[Hugh Jaleak]

Records may be kept by Cambridge City Council, or possibly Anglian Water, but ultimately highway drainage is the responsibility of the Highways Authority, be this the City or County Council. It does seem from the pics, a rather large area to be relying on one gulley to accept the runoff, especially as there appears to be no provision for alternative means of drainage (to a sewer/soakaway) should the solitary gulley cease to function....

Secondly I really cannot see that area of garages and hardstanding was designed and built without adequate provision for surface water drainage. Assuming it was built at the same time or after your property was built then it is highly unlikely the design expected the surrounding gardens to absorb the runoff. (Until recently everything was connected to the sewer. Legally if there was a sewer within a certain distance of the property boundary the developer was allowed to connect regardless of whether that sewer had the capacity to cope!)

I would firstly contact the City Council as landowner, explain the issue and ask what they propose to do about it. You may have a case against them for causing a statutory nuisance, as it is water from their paved area that is entering your property due to there being no adequate method for drainage. Throw the ball into their court and see what response you receive.

 

[freddiemercurystwin]

Hugh Jaleak, I only just got your username!

 

[evyxmsj]

Hello,

I would definitely go with Hugh's suggestion.

Try to avoid going down the route of public surface water sewer (Cambridge Water?) as they won't want to accept any new runoff without justification. It can be done if necessary though, but you will need to apply.

Highway drainage sounds like it would have more success, especially if you can get hold of any record drawings that show it should have been served by adequate drainage. This means that you are not necessarily adding impervious area to an existing system that wasn't designed for it. Even if not, you should still probably be okay as it is a small area.

If it helps, I can work up some runoff rates if you get me the impervious plan area to be drained. Just measure it out the best you can.

 

[markocosic]

Thanks folks.

House (all of Arbury) is late 1950s council housing. Garages are concrete prefabs. I think they're newer but will ask the little old lady down the way who remembers the lot being built...

Results of 6 mm of rain today:

Untitled

• markocosic
• 16 Oct 2013
• 1

Untitled

• markocosic
• 16 Oct 2013
• 1

I'll ask he highways lot (Cambridgeshire *County* Council) about that blocked drain in the morning as a good lead into "how should that lot drain anyway?" It's all owned by Cambridge *City* Council and the larger paved area is *City* Council responsibility but the bits that's public highway is *County* Council's maintenance ticket. Fun!

There's definitely no gulley/soakaway at the end of my garden though. Wall foundations and the kerbstone haunchings only, and no sign of there ever having been anything else.

Untitled

• markocosic
• 16 Oct 2013
• 1

My guess is that the line shown as "Jermyn Close" on Google Maps was built as a "road" (hence proper drain) and the rest has been tarmaced over at a later date (carpark for the adjacent flats? turning area for the garages?) without much thought to drainage. (it's a different type of tarmac)

Untitled

• markocosic
• 16 Oct 2013
• 1

Area?

Total area (excl garage roofs that drain into the line drain) is 500m^2. There's only that one square "road drain" circled in orange to drain it and some rain falls into that, but I'd say an easy 250m^2 runs into the garden.

 

[Hugh Jaleak]

Water levels in existing drainage system seem very high anyway, and now after rain there is a puddle above the drain so looks pretty certain the system is blocked. Get the appropriate body to clean the gullies out, and check the system is not bunged up downstream. There does appear to be a second gulley towards the back of the paved area in front of the garages, is this clear?

Looking at the falls it seems additional drainage maybe required to accept the runoff from the paved area rather than it finding it's way onto your property. Will involve expense, but AFAIK the Council are responsible for draining their hardstanding and not expect you to accept the runoff from the communal paved areas.

 

[markocosic]

There's one gulley on the footpath/next to the wall at the side of my property, which drains into a clay drain. This drain is clear and the (high) level is set by the "trap" in the drain. (it's a pot with a flat bottom and a side exit that goes up before going down, much like a toilet bowl shape)

There's a line drain at the entrance to the garages. This is located at the high point of the paved area and nothing except runoff from the garage roofs runs into it.

There's then that "road drain" (almost square, 18" on a side) near the southwest corner that backs up instantly during rain.

No second gulley. The strip between the paved area and my wall is full of concrete foptings/haunchings only and isn't supposed to be a drain.

Will update with what the highway drains people say; cheers for pointers!

 

[evyxmsj]

Hello,

Some runoff calcs for you in case it is useful. I have included the input variables and methodology in case you need to provide details to the Council.

Method: Manual runoff calculation for use in the Modified Rational Method(1).

FSR Data for Cambridge:
- M5-60 = 20.0 mm
- r = 0.45
Coefficients (average values)
- Cv = 0.75
- Cr = 1.3
- C = 1.3 x 0.75 = 0.975

The time of entry for your small site will be in the order of 5 min or fewer. (Time of Entry means the time it takes runoff to reach a drain).

Mean rainfall intensities (i) are predicted as follows, for a 5 min storm:
- 1 year return period (RP) = 58 mm/hr
- 2 year RP = 74.7 mm/hr
- 30 year RP = 138 mm/hr
- 100 year RP = 175 mm/hr
- 100 year plus climate change (20%) = 210 mm/hr

Using the Modified Rational Method, the peak runoff (Q) for a given storm is defined by:
- Q = CiA
(Where A is the impervious area in hectares, Q is in l/s).

Using 500 sq.m (0.05 ha), your peak runoff rates will be:
- 1 year RP = 8 l/s
- 2 year RP = 10 l/s
- 30 year RP = 19 l/s
- 100 year RP = 27 l/s
- 100 year plus climate change = 28 l/s

Generally, you will size your gully/filter drain/channel drain to collect a 1 or 2 year return period event. I'd stick with the latter unless your site is steeper than 1:100. A 150 mm pipe with a gradient of more than 1:294 will be sufficient (though generally not a good idea to go shallower than 1iameter).

The higher rates are used for checking the underground receiving drainage system for flooding. You shouldn't need to worry about it----I included it in case the Council asks. Generally, the drainage system should not flood at all for a 30 year return period. Newer systems will be checked for a 100 year return period (1% annual chance) with climate change allowance to ensure that properties and people will be safe.

I hope that is slightly helpful. Most road gullies will accept 10 l/s, and many channel drains will be more than sufficient. If you have to DIY, Marshall's drains and Aco's drains are often used in highway works.

(1) DoE, 1983. Design and Analysis of Urban Storm Drainage: the
Wallingford Procedure. Department of Environment, National
Water Council, Standing Technical Committee Report 28. Hydraulics
Research Ltd. Wallingford, Oxfordshire.

 

[markocosic]

Hello!


Spoke with Highways @ Cambridge (County) Council. They've now been, unblocked the gulley nearest my house, spotted that second gulley that I hadn't and unblocked that too. More than enough drainage for the area according to your calcs (many thanks for those!) now that those gullies are clear, and the levels within them have dropped substantially. (now ~1 metre below the surface rather than 150 mm)

There's still a fall towards my property in that one corner for whatever reason, be it f**kup on day one or settling over time. This "won't be getting fixed any time soon" but I've been told that if I build my garage to the existing levels then as/when that area is resurfaced the falls will be made to suit.

In the interim the problem isn't anywhere near as bad now that the gullies are clear, and "if an unauthorised surface water connection into the pipe for that pavement drain then this also wouldn't be getting fixed any time soon." Fairly pragmatic of them. There appears to be a gap under the kerbstone into the side of that (mostly cracked to **** anyway) P-trap that drains the water falling off the tarmac and along the top of the footings.


Longer term I'll drop one of those Marshall/Aco channel drains on my side of the boundary at the entrance to the garage. I'll need gutters for the roof anyway so combining the two isn't materially more work.

Legalities aside, would it be wiser to connect this to that clay pot (surface water/not mine) or the house system (combined/mine)? Perhaps make provision for both and block the connection to the house system until such a time as the highway/falls are corrected so that additional 100m^2 doesn't overwhelm the house drains in heavy rain?

 

[evyxmsj]

Irrespective of permissions, I wouldn't connect any new surface water drainage to a combined system if there is an alternative. That is, unless you know of a specific capacity problem on the sw system.